EP0760486A1 - Light trap - Google Patents

Abstract

The light trap, designed for a parallel light beam (2), comprises a cylindrical chamber (1A) with a side wall (3) which has an absorbent inner surface (4), and an inlet member (5) at one end of the chamber with inner (7) and outer (8) surfaces and a truncated conical aperture (9) between them. The aperture is located coaxially with the chamber and a conical body (12) located inside it and fixed to its base (10). The truncated conical aperture (9) converges from the outer face of the inlet member to its inner face, and its diameter on the outer face is equal to the diameter (D) of the light beam (2). The thickness (E) of the inlet member is determined from the equation E = D/(tg2A + tgA), where A is half the angle of the aperture. The inner conical body (12) is coated with a material which reflects light equally in all directions, e.g. barium sulphate with visible light and gold for infrared light. In addition, the outer wall is designed to dissipate heat, and is made from copper.

Description

The present invention relates to a light trap.

Generally, such a light trap is used in optical assemblies or optical instruments to eliminate stray light, which may disturb the operations to be carried out. The stray light to be eliminated is often formed by a light beam generated during said operations and not used at least in part. It may, for example, be the scattered and unused part of a light beam projected onto a semi-transparent mirror, the reflected part of which is necessary for the treatments and operations envisaged, or vice versa.

Such stray light is harmful since it is likely to disturb the processing carried out, for example by generating stray images in image processing devices, or by modifying the light intensity in instruments for measuring the light intensity. .

• une paroi latérale à surface interne absorbante ;
• une pièce d'entrée agencée longitudinalement à une extrémité de ladite enceinte, présentant une face interne et une face externe, et munie d'une ouverture traversante tronconique coaxiale à ladite enceinte, par laquelle le faisceau lumineux est susceptible de pénétrer dans l'enceinte ;
• un fond agencé longitudinalement à l'autre extrémité de ladite enceinte ; et
• un corps conique solidaire dudit fond et coaxial à ladite enceinte, dont la pointe de cône est en regard de l'ouverture tronconique.

Document FR-2 088 198 discloses a light trap intended for eliminating a stray light beam, said light trap comprising a chamber of revolution comprising:

• a side wall with an absorbent internal surface;
• an entry piece arranged longitudinally at one end of said enclosure, having an internal face and an external face, and provided with a frustoconical through opening coaxial with said enclosure, through which the light beam is capable of entering the enclosure;
• a bottom arranged longitudinally at the other end of said enclosure; and
• a conical body integral with said bottom and coaxial with said enclosure, the cone tip of which faces the frustoconical opening.

Said frustoconical opening narrows from the internal face of the entry piece towards the external face.

In addition, the entrance part comprises, on the external face around said frustoconical opening, an inclined surface intended to reflect a light radiation meeting said surface towards an absorbent peripheral surface provided opposite it.

• soit à des faisceaux lumineux qui ne sont pas centrés sur l'ouverture tronconique,
• soit à des faisceaux lumineux dont le diamètre est supérieur au diamètre de ladite ouverture tronconique.

This auxiliary light elimination means is intended for light beams, at least part of which cannot penetrate said enclosure, that is to say:

• either to light beams which are not centered on the frustoconical opening,
• or to light beams whose diameter is greater than the diameter of said frustoconical opening.

• d'une part, le rayonnement qui est traité par ledit moyen auxiliaire d'élimination de lumière n'est pas totalement annihilé ; et
• d'autre part, le rayonnement qui est traité dans l'enceinte risque d'être partiellement réfléchi vers l'extérieur de celle-ci, notamment en raison du mode d'agencement de l'ouverture tronconique.

Said known light trap has the drawback of not completely eliminating the stray light treated since:

• on the one hand, the radiation which is processed by said auxiliary light elimination means is not completely annihilated; and
• on the other hand, the radiation which is treated in the enclosure risks being partially reflected towards the outside thereof, in particular due to the mode of arrangement of the frustoconical opening.

The object of the present invention is to remedy these drawbacks. It relates to a light trap of the type described above which is formed so as to present a substantially increased efficiency.

• une paroi latérale à surface interne absorbante ;
• une pièce d'entrée agencée longitudinalement à une extrémité de ladite enceinte, présentant une face interne et une face externe, et munie d'une ouverture traversante tronconique coaxiale à ladite enceinte, par laquelle le faisceau lumineux est susceptible de pénétrer dans l'enceinte ;
• un fond agencé longitudinalement à l'autre extrémité de ladite enceinte ; et
• un corps conique solidaire dudit fond et coaxial à ladite enceinte, dont la pointe de cône est en regard de ladite ouverture tronconique,

est remarquable :

• en ce que ladite ouverture tronconique se rétrécit de la face externe de la pièce d'entrée vers la face interne pour concentrer la lumière et présente sur ladite face externe un diamètre égal au diamètre D dudit faisceau lumineux ;
• en ce que ladite pièce d'entrée présente longitudinalement à ladite enceinte une épaisseur E définie par la relation : $$\text{E = D/(tg2A + tgA),}$$
  
  tgA étant la tangente de A et A étant le demi-angle de ladite ouverture tronconique ; et
• en ce que ledit corps conique est réalisé en une matière réfléchissant la lumière de façon analogue dans toutes les directions.

To this end, according to the invention, the light trap for a parallel light beam, comprising an enclosure of revolution comprising:

• a side wall with an absorbent internal surface;
• an entry piece arranged longitudinally at one end of said enclosure, having an internal face and an external face, and provided with a frustoconical through opening coaxial with said enclosure, through which the light beam is capable of entering the enclosure;
• a bottom arranged longitudinally at the other end of said enclosure; and
• a conical body integral with said bottom and coaxial with said enclosure, the cone tip of which faces said frustoconical opening,

is remarkable:

• in that said frustoconical opening narrows from the external face of the entry piece towards the internal face to concentrate the light and has on said external face a diameter equal to the diameter D of said light beam;
• in that said entry piece has longitudinally to said enclosure a thickness E defined by the relation: $$\text{E = D / (tg2A + tgA),}$$
  
  tgA being the tangent of A and A being the half-angle of said frustoconical opening; and
• in that said conical body is made of a material reflecting light in a similar manner in all directions.

• le diamètre de l'ouverture tronconique est adapté audit faisceau lumineux de sorte que ce dernier pénètre entièrement dans l'enceinte ;
• l'épaisseur de la pièce d'entrée est choisie de manière à optimiser la concentration du faisceau lumineux à l'intérieur de l'enceinte ;
• l'ouverture tronconique, qui se rétrécit vers la face interne de la pièce d'entrée et y présente un diamètre considérablement réduit, empêche toute sortie de lumière de l'enceinte ; et
• la réflexion par le corps conique de la lumière dans toutes les directions entraîne une atténuation de l'amplitude de celle-ci, ce qui facilite son absorption par la paroi interne de l'enceinte (éventuellement après plusieurs réflexions successives sur ladite paroi interne et ledit corps conique) et donc son élimination.

Thus, thanks to the invention, the treated light beam is completely eliminated, since:

• the diameter of the frustoconical opening is adapted to said light beam so that the latter fully penetrates the enclosure;
• the thickness of the entry piece is chosen so as to optimize the concentration of the light beam inside the enclosure;
• the frustoconical opening, which narrows towards the internal face of the entry piece and has a considerably reduced diameter there, prevents any exit of light from the enclosure; and
• the reflection by the conical body of light in all directions results in an attenuation of the amplitude of this, which facilitates its absorption by the internal wall of the enclosure (possibly after several successive reflections on said internal wall and said conical body) and therefore its elimination.

Furthermore, advantageously, for a light beam formed by visible light, said conical body is coated with barium sulfate, while, for a light beam formed by infrared radiation, said conical body is coated with gold .

In addition, the side wall is advantageously formed so as to dissipate the heat. For this purpose, it is preferably made of copper, possibly coated with an absorbent material depending on the incident radiation.

The figures of the appended drawing will make it clear how the invention can be implemented. In these figures, identical references designate similar elements.

Figure 1 schematically shows a light trap according to the invention.

FIG. 2 schematically illustrates the entry piece of a light trap according to the invention.

The light trap 1 according to the invention and shown schematically in Figure 1 is intended to eliminate a parallel light beam 2 having a circular cross section of diameter D.

• une paroi latérale 3 à surface interne absorbante 4 ;
• une pièce d'entrée 5 agencée longitudinalement à une extrémité 6 de ladite enceinte 1A, présentant une face interne 7 et une face externe 8, et munie d'une ouverture traversante tronconique 9 coaxiale à ladite enceinte 1A, par laquelle le faisceau lumineux 2 est susceptible de pénétrer dans l'enceinte 1A ;
• un fond 10 agencé longitudinalement à l'autre extrémité 11 de ladite enceinte 1A ; et
• un corps conique 12 solidaire dudit fond 10 et coaxial à ladite enceinte 1A, dont la pointe de cône 13 est en regard de ladite ouverture tronconique 9.

In known manner, said light trap 1 comprises a revolution enclosure lA of axis XX comprising:

• a side wall 3 with an absorbent internal surface 4;
• an input part 5 arranged longitudinally at one end 6 of said enclosure 1A, having an internal face 7 and an external face 8, and provided with a frustoconical through opening 9 coaxial with said enclosure 1A, through which the light beam 2 is likely to enter enclosure 1A;
• a bottom 10 arranged longitudinally at the other end 11 of said enclosure 1A; and
• a conical body 12 secured to said bottom 10 and coaxial with said enclosure 1A, the cone tip 13 of which faces said frustoconical opening 9.

• l'ouverture tronconique 9 présente sur la face externe 8 de la pièce d'entrée 5 un diamètre égal au diamètre D du faisceau lumineux 2 à éliminer. Ainsi, en centrant le piège 1 sur le faisceau lumineux 2, ledit faisceau lumineux 2 pénètre entièrement dans l'enceinte 1A. Il n'existe pas de réflexion dudit faisceau 2 sur la face externe 8 de la pièce d'entrée 5 ;
• l'ouverture tronconique 9 se rétrécit de ladite face externe 8 vers la face interne 7 de la pièce d'entrée 5, de manière à présenter sur ladite face interne 7 un diamètre d réduit (représenté sur la figure 2), qui empêche une éventuelle sortie d'un rayonnement de l'enceinte 1A après réflexion interne ;
• la pièce d'entrée 5 présente longitudinalement à l'enceinte 1A une épaisseur E définie, en relation avec l'ouverture tronconique 9, de manière à optimiser la concentration du faisceau lumineux 2 à l'intérieur de l'enceinte 1A, tel que précisé ci-dessous en référence à la figure 2 ; et
• le corps conique 12 est réalisé en une matière réfléchissant la lumière de façon analogue dans toutes les directions et non de façon spéculaire. A cet effet, on utilise de préférence du sulfate de baryum lorsque le faisceau lumineux est formé d'un rayonnement visible et de l'or lorsqu'il est formé d'un rayonnement infrarouge ou proche infrarouge.

According to the invention, in order to obtain complete elimination of said parallel light beam 2, said light trap 1 has the following characteristics:

• the frustoconical opening 9 has on the external face 8 of the input part 5 a diameter equal to the diameter D of the light beam 2 to be eliminated. Thus, by centering the trap 1 on the light beam 2, said light beam 2 fully penetrates into the enclosure 1A. There is no reflection of said beam 2 on the external face 8 of the input part 5;
• the frustoconical opening 9 narrows from said external face 8 towards the internal face 7 of the entry piece 5, so as to present on said internal face 7 a reduced diameter d (shown in FIG. 2), which prevents possible radiation from the enclosure 1A after internal reflection;
• the entry piece 5 has longitudinally at the enclosure 1A a defined thickness E, in relation to the frustoconical opening 9, so as to optimize the concentration of the light beam 2 inside the enclosure 1A, as specified below with reference to FIG. 2; and
• the conical body 12 is made of a material reflecting light in a similar manner in all directions and not in a specular manner. To this end, barium sulphate is preferably used when the light beam is formed from visible radiation and gold when it is formed from infrared or near infrared radiation.

Thus, the radiation which enters the enclosure 1A is reflected in all directions by the conical body 12 so that they are greatly weakened in amplitude. Therefore, they are more easily absorbed by the internal surface 4 of the wall 3, arranged opposite said conical body 12. Of course, depending on the intensity of the radiation considered, several successive reflections on said internal surface 4 and said conical body 12 may be necessary to completely eliminate said radiation.

In addition, the side wall 3 is formed so as to dissipate the heat generated by the absorption of light. To this end, said side wall 3 is preferably made of copper, with a coating of absorbent material as a function of the incident radiation.

Thus, thanks to the aforementioned characteristics, the light trap 1 according to the invention makes it possible to completely eliminate the treated light beam 2.

Furthermore, as indicated above, the thickness E of the entry piece 5 and the dimensions of the frustoconical opening 9 are chosen jointly so as to allow an optimal concentration of light inside the enclosure 1A.

More precisely, these dimensions are chosen so that a radiation R which undergoes a reflection at the external end 14 of the opening 9 enters the enclosure lA without additional reflection, as shown in FIG. 2. Said radiation R intersects axis XX at point 15.

• A comme le demi-angle de l'ouverture tronconique 9,
• h comme la distance longitudinale (suivant l'axe X-X) entre la face interne 7 et le point 15, et
• p comme la distance longitudinale entre la face externe 8 et le point 15, c'est-à-dire E=h+p,

on obtient facilement les relations suivantes à partir de la figure 2 :

• tg2A = D/(2p), tg2A étant la tangente de l'angle 2A, qui permet d'obtenir la relation : $$\text{p = D/(2tg2A)}$$
  
• tg2A = d/(2h), qui permet d'obtenir la relation : $$\text{h = d/(2tg2A)}$$
  

By defining :

• A like the half-angle of the frustoconical opening 9,
• h as the longitudinal distance (along the axis XX) between the internal face 7 and the point 15, and
• p as the longitudinal distance between the external face 8 and the point 15, that is to say E = h + p,

the following relationships are easily obtained from Figure 2:

• tg2A = D / (2p), tg2A being the tangent of the angle 2A, which allows to obtain the relation: $$\text{p = D / (2tg2A)}$$
  
• tg2A = d / (2h), which allows to obtain the relation: $$\text{h = d / (2tg2A)}$$
  

Said relations (1) and (2) make it possible to deduce the expression: $$\text{(D + d) = 2Etg2A}$$

In addition, we have the relation tgA = (Dd) / (2E), that is to say $$\text{(Dd) = 2EtgA}$$

Finally, we deduce from the previous relations (3) and (4) the expression: $$\text{E = D / (tg2A + tgA)}$$

This last expression (5) therefore specifies the optimal thickness E according to the invention of the entry piece 5, as a function of a determined diameter D and a half-angle A.

Claims (6)

Light trap for a parallel light beam (2), comprising a revolution enclosure (1A) comprising: - a side wall (3) with absorbent internal surface (4); - An entry piece (5) arranged longitudinally at one end (6) of said enclosure (1A), having an internal face (7) and an external face (8), and provided with a frustoconical through opening (9) coaxial with said enclosure (1A), through which the light beam (2) is capable of penetrating into the enclosure (1A); - A bottom (10) arranged longitudinally at the other end (11) of said enclosure (1A); and a conical body (12) integral with said bottom (10) and coaxial with said enclosure (1A), the cone tip (13) of which faces said frustoconical opening (9), characterized: - in that said frustoconical opening (9) narrows from the external face (8) of the entry piece (5) towards the internal face (7) to concentrate the light and has on said external face (8) a diameter equal to the diameter D of said light beam (2); - in that said entry piece (5) has longitudinally to said enclosure (1A) a thickness E defined by the relation: $$\text{E = D / (tg2A + tgA),}$$

tgA being the tangent of A and A being the half-angle of said frustoconical opening (9); and - in that said conical body (12) is made of a material reflecting light in a similar manner in all directions. Light trap according to claim 1, for a light beam formed by visible light,
characterized in that said conical body (12) is coated with barium sulfate. Light trap according to claim 1, for a light beam formed by infrared radiation,
characterized in that said conical body (12) is coated with gold. Light trap according to any one of the preceding claims,
characterized in that said side wall (3) is formed so as to dissipate heat. Light trap according to claim 4,
characterized in that said side wall (3) is made of copper. Light room according to claim 5,
characterized in that said copper side wall is coated with an absorbent material depending on the incident radiation.

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